The present invention relates to the enhancement of sulfur capture during combustion or gasification of solid, sulfur-containing carbonaceous fuel, and more particularly to a method of processing a mixture of particulate coal and a sulfur absorbent in order to enhance the ability of the sulfur absorbent to capture sulfur during combustion or gasification.
Although coal is a major source of energy in the United States, it is well known that combustion of coal having a high sulfur content can produce considerable air pollution, as well as an ash containing leachable forms of sulfur constituting a health hazard. One technique for suppressing pollutants such as sulfur dioxide and leachable sulfur in the ash, resulting from combustion or gasification of high-sulfur coal, is to physically mix the coal with a sulfur absorbent, e.g., calcium oxide (CaO), calcium hydroxide (Ca(OH).sub.2) or calcium carbonate (CaCO.sub.3) prior to combustion or gasification. This technique has been used extensively in connection with fluidized bed combustion and gasification processes. Prior art workers have found, however, that satisfactory sulfur capture during fluidized bed combustion of coal-sulfur absorbent mixtures occurs only if the temperature in the fluidized bed combustor does not exceed 1650.degree. F. Thus, presentday fluidized bed combustion systems burning coal-sulfur absorbent mixtures are designed accordingly, despite the distinct advantages that could be achieved at high combustion temperatures, e.g., higher heat transfer rates and steam temperatures.
It is recognized that industry acceptance of fixed-bed type gasifiers has been barred, at least in part, because of the fact that the raw products from such gasifiers contain a liquid hydrocarbon phase, e.g., tars. Tar formation reduces carbon conversion to product gas and creates additional handling and disposal problems, thus complicating plant design and operations, and creating potential health and environmental concerns. Moreover, fixed-bed gasifiers typically include a combustion zone or zones operating at temperatures previously considered to be in excess of those required for effective sulfur capture.
U.S. Pat. No. 4,111,755 discloses a method of producing a pelletized fixed-sulfur coal or coke. A mixture of coal and a sulfur absorbent (limestone) is ground and blended and then balled or compacted to form pellets, and these pellets are then subjected to either a pyrolyzing or carbonizing technique at high temperatures within a reducing or slightly oxidizing environment to cause simultaneous high-temperature decomposition of the hydrocarbonaceous matter of the coal, i.e. removal of the volatiles, and calcination, with sulfur fixation of the basic constituents. The overall intent of the pelletizing operation is to co-react limestone particles with coal particles during pyrolysis or carbonizing so as to cause sulfur to react and fix with the lime while the coal is undergoing pyrolytic decomposition. The final result is a pellet which, because of its size and lack of volatile matter, cannot be burned in a pulverized coal furnace.
It is an object of the present invention to provide a method of enhancing sulfur capture by sulfur absorbents during combustion or gasification of particulate fuel, and to substantially reduce the caking tendencies of most coals during gasification.
A further object of the invention is to permit satisfactory sulfur capture to occur in fluidized bed combustion systems operating at temperatures in excess of 1650.degree. F.
It is a further object of the present invention to provide satisfactory sulfur capture without the need for pelletization of the coal, and without requiring pyrolysis or carbonization of the coal to remove the volatile matter and fix the sulfur.
Still another object of the present invention is to enhance the performance of fixed-bed gasification systems, and in particular, to substantially reduce or eliminate the liquid hydrocarbon phase (tars) normally included in the raw products from such fixed-bed gasifiers, and to permit sulfur capture in the combustion zone or zones of such gasifiers at temperatures considerably higher than previously thought possible.
We have also found that the process of the present invention yields a significant increase in the volatile matter present in the treated coal.